the 5-in-1 network admin's cable http://www.ossmann.com/5-in-1.html 1 de 10 30/12/2009 02:27 p.m. the 5-in-1 network admin's cable ethernet / crossover / modem / null modem / Cisco console featured in the premiere edition ofMake There is nothing worse than showing up for an on-site visit without the correct cable you need for a job. So what is a network admin to do, carry around several bulky cables everywhere? For quite some time I've been carrying a single CAT5 ethernet cable in my laptop bag along with "the world's shortest crossover cable" and an RJ45 coupler. This allows me to turn my ethernet cable into a crossover cable and eliminates the need to carry around a second lengthy cable. Handy, but nothing to write home about. More recently I have been working with a lot of network devices with serial console ports, including Cisco ro uters and various oth er things. This means luggin g around a null modem cable in addition to the non-standard RJ-45 to DB-9 console cable required by most Cisco equipment. But after putting together four simple custom adapters, I now have an ethernet cable, a crossover ethernet cable, a modem cable, a null modem cable, and a Cisco console cable all by carrying around just one standard ethernet cable. An added benefit is that I can always extend my cable by finding a longer ethernet cable than the one I carry in my bag. It is usually pretty easy to locate a long ethernet cable but not so easy to locate a long null modem cable. Do you find yourself toting several of these cables everywhere you go? Do you often wish you had brought a different c able with you after you have arrived on-site? Are you as geeky as me and think tha t this is just plain cool even if you never expect to configure a router in your lifetime? Then you need the 5-in-1 network admin's cable! Here's how to make your own: Buy, beg, borrow or steal the tools and materials you will need: 2 DB9 female to RJ45 female modular adapters These are the kind which you can pin however you want. 8 extra female pins for the DB9 connectors Plus have a few extras on hand unless you are much more dextrous than me. 4 RJ45 modular plugs Ditto on the extras just in case. 2 short lengths of CAT5 cable, preferably of different colors You can cut up existing cables. 1 RJ45 coupler1.
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rather than with each other, which should cut down on cross-talk and allow longer cable lengths. (I also
think it is nifty to be able to use the same crossover cable for both ethernet and serial.) I haven't done any
length or reliability testing, but so far everything I have tried has worked just fine.
If you need a DB9/RJ45 serial cable with different pinouts than the Cisco one, all you have to do is make
another little CAT5 adapter. Cable ends are cheap and plentiful.
When used as a serial cable, this is a hardware flow control (CTS/RTS) cable. If you are using devices
which both require hardware flow control, it should work. If neither of your devices require hardware flowcontrol, it should still work. However, if one of your devices requires hardware flow control and the other
does not support hardware flow control, then you need a cable that fakes flow control. This could be done
with an additional DB9/RJ45 adapter or with another CAT5 adapter and some creative crimping (my
preference), both of which are left as exercises for the reader.
If you grab a random ethernet cable to use with your serial adapters, remember to make sure that it has all
eight conductors. Also keep in mind that most ethernet cables have only been tested for connectivity on
four conductors (1, 2, 3, and 6) if they have been tested at all. I used those conductors for the most
important serial signals (Transmit, Receive, and Ground) just in case, but some serial devices won't talk
without all eight working.
Extras:
Many additional adapters could easily be added to this set. A few that leap to mind would be for other
kinds of serial ports such as DB-25 and various DIN and miniDIN ports for Macintoshes and other things.
If you come up with a useful one, send me the pinout (as well as a link to an authoritative source regarding
your device's pinout if possible) and I will post it here.
DB25 serial:
DB25 pin signal RJ45 pin-------- ------ --------
8 DCD 83 RxD 3
2 TxD 1
20 DTR 57 SG 2 and 66 DSR 8
4 RTS 45 CTS 7
1.
loopback plug
You can use the leftover wires in that inch of CAT5 that you cut when making the DB9 adapters.Pull four of them out and crimp both ends (in little loops) in a single RJ45 plug as follows:
1 TxD wire one2 SG wire two
3 RxD wire one4 RTS wire three5 DTR wire four
6 SG wire two
7 CTS wire three8 DSR/DCD wire four
The photograph depicts a three-wire
RS-232 loopback plug, not the four
wire combination plug.
This works for RS-232 loopback. Alert reader, Bruce Bannach, pointed out that it also works for